1. Field of the Invention
This invention relates to a process for the manufacture of acrolein or acrylic acid from propylene. More specifically, it describes an improved process for producing acrolein or producing acrylic acid by the catalytic vapor phase oxidation of propylene in the presence of essentially inert, essentially anhydrous diluents having specified composite heat capacity values.
2. Summary of the Prior Art
Generally, propylene in its gaseous phase is oxidized to acrolein in the presence of molecular oxygen-containing gases and steam, whose concentration is often as high as about 35 volume percent of the total feed stream, by contact at elevated temperatures with solid metal oxide catalysts. The acrolein produced in this reaction stage can be recovered or can be directed without separation of the acrolein to a second reactor operating in series with the first reactor to oxidize the acrolein to acrylic acid.
In the prior art, steam has been used in the starting reactant gas mixture in order to avoid flammable gas mixtures and because it was believed to be important to reaction selectivity, but the prior art does not recognize the importance of and unexpected effect imparted to the process of the composite heat capacity value of the gas, or steam, mixture used. For example, U.S. Pat. No. 4,147,885 relates that it is wide practice to incorporate steam to avoid burning the reactant gases and increase selectivity to acrylic acid. U.S. Pat. No. 3,475,488 discloses that it is desirable to incorporate steam in the starting reactant gas since this increases conversion and selectivity when employed in the order of 1 to 60, and preferably 5 to 30, moles of steam per mole of propylene or propylene plus acrolein.
Other patents also describe steam as the preferred diluent. For example, U.S. Pat. No. 3,171,859 states that "addition of steam is obligatory . . . it acts not only as a diluent, but also favors the reaction in that combustion to carbon oxides is substantially reduced." Also, U.S. Pat. No. 4,267,386 reiterates the general understanding among those skilled in the art, that while inert diluents may be added to the reaction system, "water, in the form of steam is desirably present . . . in amounts of from 0.5 to 15, preferably 2 to 15, moles per mole of unsaturated hydrocarbon (i.e., propylene or acrolein)." Again, with no recognition of composite heat capacity value of the diluent.
Many oxidation catalysts have been disclosed for producing acrolein in high yield by oxidizing propylene. Predominantly, these are catalysts containing mixed oxides of molybdenum, bismuth and iron with phosphorous or tungsten or antimony. Cobalt and/or nickel and alkali metals are common promoters.
Catalysts which have been found to be advantageous for use in oxidizing acrolein to acrylic acid at conversions of more than 98% generally contain mixed metal oxides. Such catalysts typically contain molybdenum, vanadium, tungsten, chromium, copper, niobium, tantalum and antimony.
The ultimate object of the teachings in the literature cited above is to obtain high performance catalysts which give high selectivities to acrolein and acrylic acid at high propylene conversions. Other factors which influence the economic viability or the improved performance of these processes are not considered in these prior art techniques. For example, they do not address the impact on process variables of use of high propylene concentrations, how to avoid the danger of explosion, the impact of inert reaction process feeds on recovery and waste disposal, or maintaining high catalyst performance over an extended catalyst life. These are all extremely important for commercial operation.
In commercial operation, it is of economic and ecological importance to minimize the presence of the steam which is fed to the reactors, since it passes through the system and becomes a burdensome waste water load after product recovery steps; nevertheless, to the knowledge of the present inventors, no commercial process has been successfully operated below a steam: propylene mole ratio of about 1.5:1. Furthermore, it is extremely important to minimize by-products which are difficult to separate from useful product or which carry a high economic penalty for disposal. Process improvements which will provide high catalyst performance while simultaneously maximizing propylene feedstock usage, and improvements which may promote conditions for extended useful catalyst life are important for commercial operation. Equally important is the ecological problem encountered when consideration has to be given to the disposal of millions of pounds of waste water, which will vary from about 0.5 to about 1.5 pounds of water per pound of acrolein/acrylic acid produced, generated by a single multimillion pound commercial facility for the production of acrolein and/or acrylic acid. The typical commercial plant has an annual capacity of from about 120 million to over 750 million pounds, thus giving an indication of the waste water problem. The prior art does not adequately address these issues.
U.S. Pat. No. 4,049,577 teaches an improved catalyst composition for making acrolein. The authors mention that recycle gas comprised of the noncondensable fraction of the product can be used in place of steam. They suggest that these recycled inerts are preferable to steam as diluent since they allow higher conversions of propylene and thus enable one to obtain higher yields, and also reduce the waste water load on the system; however, the use of recycled inerts is stated as being made possible by the characteristics of this particular catalyst composition. Nowhere does this patent suggest or teach that anhydrous diluents having hereinafter defined composite heat capacity values have an improved effect on selectivity or product mix, or are useful with other catalysts. The relationships between various diluents and the heat capacity effects on selectivity are not suggested.
U.S. Pat. No. 3,801,634 teaches the use of inert solids mixed with active catalysts in the first and second stage reactors used to manufacture acrolein and acrylic acid. The authors indicate that noncondensable, second-stage effluent gases can be recycled to the first stage as inert diluting gas which can, at least partly, replace steam. The authors do not show any relationship between the inert anhydrous diluent gases and improvements in product selectivity; or the desirable effect of composite heat capacity values.
U.S. Pat. No. 4,031,135 presents a recycle process in which noncondensable gases, preferably and generally including steam, are recycled to the first-stage reactor and also to the interstage (second-stage) reactor feed. There is no recognition of the benefits in using anhydrous diluents having certain composite heat capacity values with respect to their effect on yield, conversion, by-product selectivity mix and waste water generation. The authors do, however, recognize an apparent improved acrylic acid efficiency, which they attribute partly to the use of recycled off gas employed as the inert diluent. In column 4, lines 13-15 the patent says the off-gas "has been substantially freed from condensable products, including water, and essentially consists of nitrogen and small amounts of" other named compounds. In column 6, lines 45 to 54 the general composition, in volume percent of the off-gas is stated in broad and "especially" terms as being:
______________________________________ broad especially ______________________________________ propylene 0-1.5 0.2-1 oxygen 0-5 1-4 CO/CO2 0-10 1-7 acrolein 0-1 0.1-0.5 steam 0-10 0.5-5 others 0-0.1 0.01-0.05 nitrogen 100-74 97.19-81.45 ______________________________________
Further, in the only two examples presented in support of their invention the patentees specifically disclose the presence of 2% by volume and 8.9% by volume of steam in the recycled off-gas stream clearly not an essentially anhydrous diluent off-gas stream. The patent clearly teaches the recycle of nitrogen, in impure form, and steam in the off-gas to the reactors. In the new invention described in this specification the essentially inert diluent gas feed is an essentially anhydrous diluent gas composition, as hereinafter described. The gas feed differs significantly from the teachings of U.S. Pat. No. 4,031,135, it is essentially anhydrous. It is further to be noted that the patentees do not adequately establish improved efficiency to acrylic acid production, and that they do not recognize or disclose the effect of composite heat capacity of the diluent on product selectivities.
U.S. Pat. No. 4,365,087 refers to the recycling of dewatered residue gas, containing both inert and reactive gases, to increase the concentration of acrylic acid recovered. However, the authors not only consider this procedure unsatisfactory since the composition of the residue gas fluctuates but have no recognition of the concept of composite heat capacity of the diluent and its effect on the process.
U.S. Pat. No. 4,442,308 teaches the use of inert gases as diluent in the acrolein process; however, it specifies their use for a particular supported first-stage acrolein catalyst. Most common commercial catalysts for propylene oxidation to acrolein are neat (unsupported) and do not follow this patent's prescribed preparation. This patent also claims that 0.5 to 7 mole % steam is beneficial and its use is recommended. Nowhere in this patent do the authors teach the advantage of anhydrous diluents on product mix nor do they mention composite heat capacity or flowing heat capacity as major variables in controlling product selectivity to advantage and formation of undesired product streams.
U.S. Pat. No. 4,456,006 teaches a catalyst preparation for the propylene-to-acrolein reaction. It shows that nitrogen diluent presents an improvement over steam diluent when used with this catalyst. It does not recognize or disclose the composite heat capacity effect of diluent on product selectivity, nor does it show by-product and waste water reductions when using anhydrous diluents.
U.S. Pat. No. 3,717,675 describes a process for recovery of acrylic acid where acrolein is expelled from the aqueous acid collected and returned to the reactors to increase subsequent yields of acrylic acid. This patent mentions the use of inert diluents such as carbon oxides and nitrogen, but does nothing to demonstrate their importance. In fact, it states that it is necessary to add steam to the reaction in order to increase selectivity. This addition of steam, however, only serves to aggravate the waste water disposal problem.
UK Patent 2,068,947 teaches a process for producing methacrolein and methacrylic acid whereby inert anhydrous diluent gases are used, also combined with water vapor, to produce a product with a reduced quantity of condensables compared to the typical steam diluent process. The authors fail to recognize the relationship between anhydrous diluents and acetic acid reduction, and they do not address composite heat capacity of the diluent or selectivity improvements resultant from use of various anhydrous diluents.
U.S. Pat. No. 4,147,885 describes a recycle process in which steam is an essential ingredient. The object of the patented invention is to recycle steam to the reactors. This is contrary to the techniques of the instant invention, since it has now been found that the reduction or absence of added steam to the reactors is beneficial.
U.S. Pat. No. 4,618,709 presents an attempt to remedy, or at least alleviate, the waste water problem common to the existing catalytic oxidation processes for producing methacrolein and methacrylic acid from, e.g., isobutylene. This is accomplished in this patent by evaporating the waste water solution and subjecting the waste water vapor to combustion with molecular oxygen, whereby the amount of liquid waste water discharged is reduced. As can be seen, this is an expensive procedure since it involves two additional costly steps. In discussing the isobutylene oxidation process the patent refers to the well known and common procedure of carrying out the oxidation in the coexistence of an inert gas for dilution (column 1, lines 22-35 and 49-53) to control temperature and prevent explosion. The patent then mentions "nitrogen, water vapor, exhaust gases, etc." as examples of inert gases added and further states water vapor as being the most frequently employed, in an amount as great as 10 to 50 moles of water, or water vapor, added per mole of methacrolein or its precursor (column 1, lines 54-58 ). These figures clearly evidence the intentional addition of significant quantities of water to the oxidation reaction, water that becomes contaminated with reactants and products of the reaction and must subsequently be disposed of in an ecologically accepted mode. The reference nowhere suggests or discloses the importance of using an inert gas having a composite heat capacity of certain value for dilution. Nor does the reference recognize or suggest the importance of avoiding the intentional addition of supplemental quantities of water to the reaction system.
UK Patent Specification 939,713 disclosed one of the earliest catalytic processes for preparing unsaturated monocarboxylic acids from olefins. In these early processes yields of acrolein and acrylic acid were low, as evidenced by the figures presented in the examples that show low conversion, and overall yields of less than about 55% from propylene charged. By comparison, today's processes operate at exceptionally high yields, and conversions that typically approach 95% to 98%. On page 2, lines 4 to 19, this UK Specification refers to the starting materials and indicates they need not be in a pure state and may contain quantities of paraffinic hydrocarbons, such as propane or butanes. It is to be noted it is present in the starting olefin reactant as an impurity and it is not intentionally additionally introduced into the reactant as a diluent. It is also noted that its quantity is nowhere clarified and that its function is described as an entraining agent; there is no recognition of its possible use as a medium for heat removal, nor does the reference state or suggest that the hydrocarbons have any effect on maintaining the desired temperature range. Though the reference states the process can be operated in the absence of water (page 2, lines 92-94), this statement is both immediately preceded and followed by the statement that water is preferably used in quantities of from 1 to 10 moles of water, preferably 3 to 7 moles,for each mole of olefin initially introduced into the first reaction zone and all of the examples use significant amounts of water. The reference does state the reaction can be carried out in the absence of water but it nowhere indicates the water must be replaced. It merely states do not add the water. Any attempt to carry out this reaction without any added diluent would be catastrophic. In Examples I and II, 4.2 moles of water were intentionally added per mole of propylene, in Example III, 5.8 moles of water were intentionally added per mole of propylene and in Examples IV and V,12 moles of water were added per mole of propylene. Thus, water comprised the majority of the bulk of the materials introduced in all examples. Further, nowhere in this UK Specification is there any mention of the use of any other coolant or temperature control medium. Nor is there any suggestion or recognition of the importance of the composite heat capacity of the gas diluent and its effect on conversion and yield.
None of the prior art suggests or recognizes the use of various inert anhydrous diluents in specific proportions so as to have the hereinafter defined composite heat capacity that will favorably affect the product mix obtained when using any of the commonly used catalysts.
As has been indicated above, the basic two-stage process for oxidizing propylene to acrylic acid via acrolein is well known and has been extensively described in the literature. It is also known that wet, overhead gases (noncondensables) from the acrylic acid scrubber can be recycled to the first reactor stage. By this recycling of unreacted propylene and acrolein, it is predictable that an improvement in overall yield is obtained in any chemical reaction. By use of such a recycle stream, it is also possible to provide a supplemental means of controlling the steam content to the first-stage reactor, as is taught in U.S. Pat. No. 4,147,885. In the process of that patent, the steam content of the first-stage feed is required to be 4 to 30% by volume, with all the steam, except that in the starting reactant gas mixture, being provided by the recycle stream. As discussed above, however, the presence of even as little as 4% steam is disadvantageous. This finding has not been addressed, nor even identified, by the prior art.
As has also been indicated above, nowhere in the prior art is there any disclosure or suggestion of the important role exerted on the process by the composite heat capacity of the diluent gas mixture and of the unexpected and unpredictable effect exerted by the composite heat capacity of said mixture on yield, conversion, by-product formation and waste water generation.
Application Ser. No. 886,562, filed Jul. 17, 1986, describes a process for improving the prior art processes by use of an inert diluent gas having certain composite heat capacity properties. While that process provides substantial improvements in selectivity and water load, it has been found to have the draw-back of reducing second-stage (acrolein-to-acrylic acid) catalyst activity. According to the present invention, there is provided a method for correcting that draw-back.